One known type of high speed projectile accelerator is classified as a rail gun. Rail guns are characterized as having two parallel, electrically conducting, elongated metallic rails spaced from each other by solid dielectric blocks. Between the rails is an insulating gap where a projectile is located. The rails are connected to opposite terminals of a DC power supply. Current from the power supply traverses a first one of the rails, flows through an armature including a metal region in the projectile or a plasma designed to be immediately behind the projectile and then flows through the second rail back to the power supply. The current flowing through the armature between the rails produces a magnetic field, which in urn produces a mechanical force in the elongated direction of the rails. The mechanical force accelerates the projectile along the length of the rails, typically to maximum speeds on the order of about 5 kilometers per second.
Solid armatures for rail gun projectiles are either solid homogeneous conducting metal blocks or laminated armatures including alternating conducting and insulating layers extending in a direction perpendicular to the acceleration direction of the projectile along the rails. It has been previously realized that laminated armatures are probably preferable to homogeneous armatures because the laminated armatures more readily distribute current uniformly throughout the armature volume. It has been previously shown that uniform current distribution amongst the conductors of a laminated rail gun projectile traversing copper rails can be obtained if: ##EQU1## where: 2a is the gap between the rails in centimeters,
l is the length of the armature in centimeters, PA1 U.sub.km/sec is the average velocity of the projectile as it traverses the rails in kilometers per second, PA1 .sigma..sub.arm is the average conductivity of the conducting and insulating layers in the rail gun armature, and PA1 .sigma..sub.Cu is the conductivity of copper rails.
The many thin layers of the laminated armature can be viewed as a continuous anisotropic conductor having zero conductivity in the conducting layers in a direction at right angles to the gap between the rails, i.e., the conductivity of the armature in the direction of projectile movement is zero. If the inequality of Equation 1 is not satisfied, current is concentrated in the trailing outer corners of the armature when the projectile is accelerated to high velocity, i.e., a velocity in excess of 5 kilometers per second. Current concentration in the trailing armature outer corners causes extensive ohmic heating in the corners, leading to melting and evaporation of metal and/or dielectric from the corners The evaporation leads to vapor trails behind the projectile, between the rails. The vapor trail provides a medium for breakdown between the rails behind the projectile to shunt current away from the armature, to prevent efficient coupling of current from the rails into the armature.
The inequality of Equation 1 can be satisfied if the armature conductivity is smaller (less than 0.1) than the conductivity of the copper rails. However, such low conductivity layers are not suitable for the projectile armature because the entire armature volume is ohmically heated. The ohmic heating is often sufficiently great to melt the entire laminated armature before the projectile reaches a high velocity, on the order of 5 kilometers per second.
The aforementioned problems with solid armatures have caused plasma arc armatures to be investigated. The plasma arc is established directly behind the projectile, such that the current flowing in the plasma between the rails produces a mechanical force that acts on the rear of the projectile. The plasma arc typically has a conductivity of about 10.sup.-3 times that of copper so that uniform current distributions occur and the solid projectile is not subject to melting because it can be insulated so that no appreciable current flows through it. However, the surfaces of the dielectric spacer blocks between the rails have a tendency to be evaporated due to contact with the hot plasma armature arc, typically at a temperature of between 20,000.degree. K. and 50,000.degree. K. Further, the armature plasma has a very high pressure, in the kilobar range, tending to adversely affect the mechanical stability of the spacer blocks. Further, secondary arcs are frequently formed between the rails at distances somewhat removed from the base of the projectile. The secondary arcs shunt current away from the primary arcs, to lower the device efficiency. The rails also have a tendency to be melted by small subarcs in the armature structure.